Rare-earth diphthalocyanines are known from prior publications to have electrochromic properties in which the color of the diphthalocyanine can change over a period of about eight seconds upon application of a potential difference across an electrochemical cell having a diphthalocyanine film on one of the electrodes. P. N. Moskalev and I. S. Kirin, "Effect of the Electrode Potential on the Absorption Spectrum of a Rare-Earth Diphthalocyanine Layer," Opt. i Spektrosk, 29, 414 (1970) and P. N. Moskalev and I. S. Kirin, "The Elecrochromism of Lathanide Dipthalocyanines," Russian J. Phys. Chem., 46, 1019 (1972).
In U.S. Pat. No. 4,184,751 issued to M. M. Nicholson, an inventor herein, and assigned to Rockwell International Corporation, the assignee herein, there is described the use of metal diphthalocyanine complexes as the electrochromically active material in an electrochromic display cell. Rapid color changes in less than 50 milliseconds are achieved, thus alleviating the slow switching time previously reported for rare-earth dipthalocyanine complexes. Power requirements are small because of the low power switching characteristics of the display material and because the display exhibits an open circuit memory of from several minutes to several hours, depending on its construction. A multi-color, i.e., more than two color, display is achieved through use of a range of voltages applied between display and counter electrodes. Color reversal of displayed information and the background against which it is displayed is achieved through use of display electrodes in the background portions of the viewing area as well as in the character segments.
In a simpler type of display device, where color reversal is not required, the background portions of the viewing area are often provided with deposits of the display material surrounding and conforming to the outlines of the segmented character electrodes. This feature is intended to provide a uniform appearance to obscure the character electrodes when the display device is in the erased condition.
Initially, in such a device, the metal diphthalocyanine display material such as, for example, lutetium diphthalocyanine has a bright green color in both the background regions and on the character or display electrodes. Upon electrical cycling, however, the display material on the display electrodes typically does not return to the precise initial color. Instead, it exhibits an olive-green color when cycled to the erased condition. This is objectionable in a display device. When the cycled olive-green display material no longer matches the bright-green background material, confusion in reading the display can occur.
Several approaches have been proposed for use in solving the problem of the failure of the erased display electrodes to match the color of the background. First, the display device could be built to provide electrical switching of the background material, more or less as in the multi-color display mentioned above. However, since the display material in the background area ordinarily needs to be cycled less than that in the character area, and possibly only cycled once, the provision of a separate circuit to accomplish this seems a relatively expensive solution wherein the described simplicity is lost. Second, the background area of the substrate on which the display material is disposed could be tinted so that the color mix of the tint and the uncycled bright green display material would approach the desired olive-green. However, this would require the use of a mask and would thus introduce a costly step in the process. Also, the color obtained in this manner could only approximate the olive-green of the cycled display material on the character electrodes. This could result in a slight visual mismatch. As a third approach, a two-step chemical oxidation and reduction treatment could be used to cycle the bright-green background display material to olive-green, but this alternative also would require relatively complicated, less economical processing.